Discount telephone billing plans have become commonplace. In a typical plan, a customer receives a discount on all calls made if total usage exceeds a predefined time or monetary limit. However, such billing plans apply separately to individual phone lines. Thus, a customer with several individual phone lines must subscribe separately to a billing plan for each line, and must qualify for any discount separately on each phone line. It would be useful if a customer could aggregate usage on two or more phone lines at the same or different addresses, so as to receive greater benefits from a discount calling plan.